1. Field of the Invention
The present invention relates to a method for curing optical glass or thermoplastic fiber coatings and inks, and in particular, for curing pigmented coatings and inks using low power electron beam radiation.
2. Background Information
Glass or thermoplastic optical fibers are normally coated with an inner primary coating (or "primary coating") and an outer primary coating (or "secondary coating") to protect the properties of the fiber. Coated fiber can be further coated with ink layers to aid in fiber identification. Alternatively, the outer primary coating can be colored to aid in fiber identification. Additional radiation-curable coating compositions, referred to as matrix materials, can be applied to a group of coated optical fibers and then cured to form a protective anchoring structure containing bundles or ribbons of optical fibers. Individual fiber identification can be important when multiple fibers are placed in ribbon or cable structures.
Additional radiation-curable matrix materials can be used to anchor the coated optical fibers in a cable structure containing bundles or ribbons of optical fibers.
Optical fiber coatings, inks and matrix materials are normally cured after application to the fiber. Cure can be achieved by a variety of means including heat (thermal initiation) and light (photoinitiation). However, thermal initiation generally results in slower cure, and fast cure speed is very important in fiber production. Therefore, photoinitiation is generally preferred. In particular, ultraviolet light cure is generally used to achieve rapid cure. It is customary to use a photoinitiator such as an acyl phosphine oxide derivative to increase cure speed.
However, curing of pigmented coating layers such as colored outer primary coatings or inks pose special problems which do not exist for colorless coatings. For example, the presence of pigments limits the ability of light to penetrate the uncured resin and efficiently cure the resin. The effectiveness of photoinitiation can be especially reduced when the resin contains a high pigment density.
Electron beam cure of optical fiber coatings is an alternative to ultraviolet cure and has been disclosed in, for example, U.S. Pat. No. 4,581,407. Electron beam cure can be advantageous because it does not require the presence of photoinitiators which can cause coating discoloration. Both pigmented and unpigmented coatings can be advantageously subjected to electron beam cure. However, electron beam radiation can damage an underlying fiber substrate, in particular, a glass fiber substrate, by ionizing metal atoms in the glass which generates colored centers and increases attenuation of the signal transmitted therein. Electron beams can also damage the resin and adversely affect the coating's mechanical properties. Hence, this method can also have significant disadvantages.
The art has seemingly not yet provided a method of electron beam curing optical fiber coatings, and in particular colored coatings and ink layers, which does not result in damage to the underlying fiber substrate.